b and c are vectors lying on the the plane. Take a sheet of paper, draw two vectors on it, like b and c in the first figure.
Lift the paper. It is a piece of a plane (second figure). Choose a point as origin somewhere outside the plane (O), and connect it to A, B, C. ##\vec {OA}##, ##\vec {OB}##, ##\vec {OC}## are the position vectors pointing to A, B, C, points of the plane .